The mechanistic target of rapamycin (mTOR) signaling pathway integrates both intracellular and extracellular signals and serves as a central regulator of cell metabolism, growth, proliferation and survival. Discoveries that have been made over the last decade show that the mTOR pathway is activated during various cellular processes (e.g. tumor formation and angiogenesis, insulin resistance, adipogenesis and T-lymphocyte activation) and is deregulated in human diseases such as cancer and type 2 diabetes. These observations have attracted broad scientific and clinical interest in mTOR. This is highlighted by the growing use of mTOR inhibitors [rapamycin and its analogues (rapalogues)] in pathological settings, including the treatment of solid tumors, organ transplantation, coronary restenosis and rheumatoid arthritis.
In particular, mTOR complex 1 (mTORC1) positively regulates cell growth and proliferation by promoting many anabolic processes, including biosynthesis of proteins, lipids and organelles, and by limiting catabolic processes such as autophagy. Much of the knowledge about mTORC1 function comes from the use of the bacterial macrolide rapamycin. Upon entering the cell, rapamycin binds to FK506-binding protein of 12 kDa (FKBP12) and interacts with the FKBP12-rapamycin binding domain (FRB) of mTOR, thus inhibiting mTORC1 functions (reviewed by Guertin and Sabatini, 2007). In contrast to its effect on mTORC1, FKBP12-rapamycin cannot physically interact with or acutely inhibit mTOR complex 2 (mTORC2)(Jacinto et al., 2004; Sarbassov et al., 2004). On the basis of these observations, mTORC1 and mTORC2 have been respectively characterized as the rapamycin-sensitive and rapamycin-insensitive complexes. However, this paradigm might not be entirely accurate, as chronic rapamycin treatment can, in some cases, inhibit mTORC2 activity by blocking its assembly (Sarbassov et al., 2006). In addition, recent reports suggest that important mTORC1 functions are resistant to inhibition by rapamycin (Choo et al., 2008; Feldman et al., 2009; Garcia-Martinez et al., 2009; Thoreen et al., 2009). Therefore, selective inhibition of mTORC1 would enable the treatment of diseases that involve dysregulation of protein synthesis and cellular metabolism. Furthermore, this detailed understanding of regulating mTORC1 activation pathways will permit the discovery of new strategies for regulating abnormal disease processes by modulating mTORC1 activity across its spectrum of function.
Many diseases are associated with abnormal cellular responses triggered by events as described above. These diseases include, but are not limited to, autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease, and hormone-related diseases.
The mechanistic target of rapamycin complex 1 (mTORC1) is a master growth regulator that senses diverse environmental cues, such as growth factors, cellular stresses, and nutrient and energy levels. When activated, mTORC1 phosphorylates substrates that potentiate anabolic processes, such as mRNA translation and lipid synthesis, and limits catabolic ones, such as autophagy. mTORC1 dysregulation occurs in a broad spectrum of diseases, including diabetes, epilepsy, neurodegeneration, immune response, suppressed skeletal muscle growth, and cancer among others (Howell et al., (2013) Biochemical Society transactions 41, 906-912; Kim et al., (2013) Molecules and cells 35, 463-473; Laplante and Sabatini, (2012) Cell 149, 274-293). Accordingly, there remains a need to find protein kinase inhibitors useful as therapeutic agents.
Additionally, Glucose Transporters (GLUT) are a family of membrane proteins (GLUT1, 2, 3, 4, and 5) that facilitate the transport of glucose and other hexoses across cell membranes. The transport of glucose into cells is one of the most important cellular transport events because of the role in maintaining normal cellular respiration and metabolism (Gould and Holman, (1993) Biochem J., 295, 329-341). Dysfunction or dysregulation of glucose transporters may contribute to, or directly result in, disease states because of the central role the transporters play in cellular homeostasis and metabolism. For example, mutations in the GLUT1 gene are responsible for the rare autosomal disorder De Vivo disease, which is characterized by impaired glucose transport into the brain. Relatedly, elevated levels of GLUT1 in neutrophils has been found to contribute to the inflammatory response in cystic fibrosis (CF) patients (Laval et al., (2013) J. Immunol, 190(12), 6043-50). GLUT inhibition may normalize cellular metabolism and response in affected cells, including immune cells such as neutrophils. Therefore, GLUT inhibition would enable the treatment of cystic fibrosis, as well as autoimmune diseases characterized by abnormal GLUT expression or activity.